Fix failures related to at queries.

[marcandre]

1. assert_has gave an unexcepted error.
2. find gave a wrong error messages always indicating a 0 found count:
   "The element at index X is not available because 0 elements..."

This PR provides a more instructive error message for failures. It also
distinguishes between invalid values for at and values that are not found.

[marcandre]

To avoid conflicts, this includes #642 (and thus a few other PRs too). Just the last commit is new.

[codecov-commenter]

Codecov Report

Merging #647 (8fe55b0) into main (5e02d94) will increase coverage by 5.05%.
The diff coverage is 100.00%.

❗ Current head 8fe55b0 differs from pull request most recent head 29075ab. Consider uploading reports for the commit 29075ab to get more accurate results

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
+ Coverage   88.04%   93.09%   +5.05%     
==========================================
  Files          24       24              
  Lines         995      999       +4     
==========================================
+ Hits          876      930      +54     
+ Misses        119       69      -50     

Flag	Coverage Δ
IntegrationTest	88.68% <95.23%> (+7.48%)	⬆️
UnitTest	42.24% <85.71%> (+0.43%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/wallaby/browser.ex	94.54% <100.00%> (ø)
lib/wallaby/query.ex	98.50% <100.00%> (+0.04%)	⬆️
lib/wallaby/query/error_message.ex	89.47% <100.00%> (+2.20%)	⬆️
lib/wallaby/selenium.ex	76.82% <0.00%> (+59.75%)	⬆️

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[marcandre]

(It may not be obvious, but I'm fixing a bunch of little things as a prelude to fix refute_has correctly...)